Transmission systems are known whereby a prime mover, such as an internal combustion engine, drives a torque converter which in turn drives a gearbox having multiple ratios. When such a transmission is used on a vehicle, the gearbox will typically have a plurality of forward gears and one or more reverse gears. The advantage of the torque converter is that the torque being transmitted from the prime mover is multiplied. The disadvantage is that the torque converter itself absorbs energy, which energy cannot therefore be used to propel the vehicle.
In order to overcome this problem torque converters can be fitted with lock-up clutches. US2003/0186768 shows an example of torque converter with a lock-up clutch. The torque converter has a casing to which is attached an impeller. Rotatable relative to the casing is a turbine. A friction clutch operably acts between the turbine and an inside part of the casing to prevent rotation of the turbine relative to the casing under certain circumstances.
Typically the lock-up clutch is used in the high gears, for example top gear, when shock loads from the wheels of the vehicle to the torque converter are less significant. When the vehicle is used in a low gear, for example bottom gear, then the shock loads coming from the wheels are correspondingly higher and accordingly the torque capacity of the lock-up clutch may not be sufficient to withstand these loads. Thus, whilst a lock-up clutch reduces wasted energy in the high gears, it may not be able to reduce wasted energy in the low gears. Further, providing a lock-up clutch within a torque converter casing is expensive. In particular the valves used to engage and disengage the lock-up clutch are expensive.
Whilst it is possible to provide multiplate lock-up clutches in a torque converter casing, so as to allow the torque converter to lock-up in all gears, such multiplate lock-up clutches are considerably more expensive than the single plate lock-up clutch shown in US2003/0186768.